1. Field of the Invention
The present invention relates to an apparatus and method for in-furnace reduction of nitrogen oxide emissions in flue gas.
2. Description of the Prior Art
During combustion of fuels with fixed nitrogen such as coal, oxygen from the air may combine with the nitrogen to produce nitrogen oxides. At sufficiently high temperatures, oxygen reacts with atmospheric nitrogen to form nitrogen oxides. Production of nitrogen oxide is regarded as undesirable because it results in acid rain, smog and ozone formation. Furthermore, the presence of nitrogen oxide in a furnace flue gas causes the condensed gases to become corrosive and acidic. There are numerous government regulations which limit the amount of nitrogen oxide which may be emitted from a combustion furnace. Title I and IV of the Clean Air Act Amendment of 1990 place stringent limits on nitrogen oxide emissions from large power plants. Consequently, there is a need for apparatus and processes which reduce the nitrogen oxide emissions in furnace flue gas.
Numerous attempts have been made to develop apparatus and processes which reduce the nitrogen oxide emissions in a furnace flue gas. One such approach is a process known as in-furnace nitrogen oxide reduction, reburning, or fuel staging. In reburning, coal, oil, or gas is injected above the primary flame zone to form a fuel-rich zone. A reburn zone stoichiometry of 0.90 is considered optimum for NO.sub.x control. The flue gas temperature in this zone is typically in the range 2400.degree.-2800.degree. F. The reaction time is of an order of 0.5 to 1.0 seconds. In this zone, part of the nitrogen oxides are reduced to ammonia (NH3) and cyanide-like fragments (HCN, etc) and molecular nitrogen (N2). Subsequently, air is injected to complete combustion. The reduced NH3 and cyanide-like fragments are then oxidized to form N2 and nitrogen oxide (NO).
Several problems occur when this process is used. First, coal may be an inefficient reburn fuel because of its high fixed-nitrogen composition. The fixed nitrogen introduced at this location in the furnace will have less chance of being converted to N2 and therefore have a higher chance of ending up as nitrogen oxide and may, depending on the nitrogen oxide concentration of the flue gas, increase the emissions of nitrogen oxide.
Furthermore, the fuel must be injected with a sufficient volume of gas to ensure mixing. This gas can be air or recirculated flue gas. If air is used as this gas, there must be enough fuel to consume the oxygen both in the flue gas and the air, and to supply an excess of fuel so reducing conditions exist. This increases the amount of fuel which must be used as reburn fuel. The necessity of using carrier air or recirculated flue gas requires extensive duct work in the upper part of the furnace.
Additionally, the reburn fuel must be injected well above the primary combustion zone of the furnace so that it will not interfere with the reactions taking place therein. However, this fuel must be made to burn out completely without leaving a large amount of unburned carbon. To do this, the fuel must be injected in a very hot region of the furnace some distance from the furnace exit. In addition, rapid mixing of the reburn fuel with the NO containing flue gas is beneficial. The exit temperature of the furnace must be limited in order to preserve the heat exchangers surface. Therefore, a tall furnace is required to complete the second stage process.
Moreover, the fuel must be injected in such quantities as to make the upper furnace zone fuel-rich. This fuel is supplied in excess to the amount of air in the furnace and ultimately requires more air to be completely combusted. Thus, air must be injected above the reburn fuel injection. The temperature in the combustion completion zone is typically in the range of 2200.degree.-2400.degree. F. The air addition system is designed to complete burnout before entering the convective steam surfaces. This requires even more duct work and furnace volume. Consequently, the combustion completion air injection must also be designed for rapid air mixing with the flue gas.
Most coal furnaces which are now in operation are not designed to accommodate the above described methods. Major modifications such as the provision of extensive ductwork and addition of a second stage to the process are required to utilize the prior art method. Such retrofitting is expensive.
Consequently, there is a need for a combustion apparatus and process which will reduce nitrogen oxide emissions in flue gas and which can be readily used in existing furnaces. An improved reburn technology has been patented by Breen et al. in U.S. Pat. Nos. 4,779,545; 5,078,064 and 5,181,475. The new technology, called reducing eddy after burn (REAB), differs from the standard reburn described above in several respects. REAB uses less natural gas than standard reburn. The furnace is not made overall fuel-rich as it is in standard reburn. Natural gas is injected at lower temperatures, 2100.degree.-2400.degree. F., consistent with chemical kinetics. This temperature range is much lower than the temperatures used in standard reburn. Operating at lower temperatures enables potentially higher NO.sub.x reductions because the thermodynamic equilibrium NO.sub.x is less than 100 ppm at 1600.degree. F.
Breen et al. inject natural gas as fuel eddies (as generated by a turbulent fuel jet, a vortex ring generator or diffusive devices) whereas standard reburn uses turbulent gas jets with or without flue gas recirculation. REAB does not use flue gas recirculation. NO.sub.x reduction in the REAB process occurs in locally fuel rich zones, such as fuel eddies and vortex rings, in contrast to a globally fuel rich zone used in standard reburn.
Slow or controlled mixing of natural gas with flue gas occurs in REAB, in contrast to rapid mixing in standard reburn. In the REAB process there is no need for completion air addition since the furnace is over all fuel lean. Mix out (destruction) of the fuel rich zones with the flue gas occurs due to the existing turbulence in the flow. Finally, REAB is less expensive than standard reburn because it uses less natural gas, does not require flue gas recirculation; and does not require completion air.
The key differences from standard reburn are injection of natural gas in a lower temperature zone, controlled or slow mixing of natural gas with flue gas, overall fuel-lean stoichiometry (local fuel rich zones exist in fuel eddies), lower natural gas usage, and no completion air.
The present invention is an improvement over the REAB technology.